Cathode active material including lithium transition metal phosphate particles, preparation method thereof, and lithium secondary battery including the same

ABSTRACT

Provided are a cathode active material including lithium transition metal phosphate particles, wherein the lithium transition metal phosphate particles include a first secondary particle formed by agglomeration of two or more first primary particles, and a second secondary particle formed by agglomeration of two or more second primary particles in the first secondary particle, and a method of preparing the same. 
     Since the cathode active material according to an embodiment of the present invention may include first primary particles and second primary particles having different average particle diameters, the exfoliation of the cathode active material from a cathode collector may be minimized and performance characteristics, such as high output characteristics and an increase in available capacity, of a secondary battery may be further improved. In addition, since the first secondary particles are porous, the secondary particles are collapsed and fractured due to rolling when used in a cathode. Thus, a spring back phenomenon may be reduced, and as a result, adhesion and processability may be further improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 14/519,500, filed on Oct. 21, 2014, which is a continuation ofInternational Application No. PCT/KR2014/007577 filed Aug. 14, 2014,which claims priority from Korean Application No. 10-2013-0102173 filedAug. 28, 2013, all of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a cathode active material includinglithium transition metal phosphate particles, a preparation methodthereof, and a lithium secondary battery including the cathode activematerial.

BACKGROUND ART

Demand for secondary batteries as an energy source has beensignificantly increased as technology development and demand withrespect to mobile devices have increased. Among these secondarybatteries, lithium secondary batteries having high energy density, highvoltage, long cycle life, and low self-discharging rate have beencommercialized and widely used.

A carbon material has been mainly used as an anode active material ofthese lithium secondary batteries, and the use of lithium metal, asulfur compound, a silicon compound, and a tin compound has also been inconsideration. Also, lithium-containing cobalt oxide (LiCoO₂) has beenmainly used as a cathode active material, and in addition, the use oflithium-containing manganese oxide, such as layered structure LiMnO₂ andspinel structure LiMn₂O₄, and lithium-containing nickel oxide (LiNiO₂)has also been in consideration.

Since LiCoO₂ has excellent various properties such as cyclecharacteristics, LiCoO₂ has currently been widely used. However, LiCoO₂has low safety, is expensive as a raw material due to the resource limitof cobalt, and has limitations in being largely used as a power sourcefor various applications such as electric vehicles. LiNiO₂ may havedifficulty in being used in an actual mass production process at areasonable cost due to the characteristics according to themanufacturing method thereof, and lithium manganese oxides, such asLiMnO₂ and LiMn₂O₄, may have poor cycle characteristics.

Thus, a method of using lithium transition metal phosphate as a cathodeactive material has recently been studied. Lithium transition metalphosphate is broadly classified into NASICON-structured Li_(x)M₂(PO₄)₃and olivine-structured LiMPO₄, and is studied as a material havingexcellent high-temperature safety in comparison to typical LiCoO₂.Currently, Li₃V₂(PO₄)₃ has been known among NASICON-structuredcompounds, and LiFePO₄ and Li(Mn,Fe)PO₄ have been most widely studiedamong olivine-structured compounds.

In particular, since LiFePO₄, as a material having a voltage of about3.5 V vs. lithium, a high bulk density of 3.6 g/cm³, and a theoreticalcapacity of 170 mAh/g, among the olivine-structured compounds has betterhigh-temperature stability than cobalt (Co) and uses inexpensive iron(Fe) as a raw material, it is highly possible for LiFePO₄ to be used asa cathode active material for a lithium secondary battery in the future.

However, since LiFePO₄ has the following limitations, LiFePO₄ haslimitations in commercialization.

First, since LiFePO₄ has low electrical conductivity, the internalresistance of a battery may increase when the LiFePO₄ is used as acathode active material. As a result, polarization potential increaseswhen a battery circuit is closed, and thus, battery capacity may bereduced.

Second, since LiFePO₄ has a lower density than a typical cathode activematerial, there is a limitation that the energy density of the batterymay not be sufficiently increased.

Third, since an olivine crystal structure in a state in which lithium isdeintercalated is very unstable, a movement path of lithium ions isobstructed by a portion in which lithium is removed from the surface ofa crystal. Thus, the intercalation/deintercalation of lithium may bedelayed.

Therefore, a technique has been proposed in which a discharge capacityis increased by decreasing the moving distance of lithium ion byreducing the diameter of the olivine crystals to a nanoscale level.

However, in the case that an electrode is prepared by using olivineparticles having a fine particle size, exfoliation from a cathodecollector may be facilitated due to a spring back phenomenon, and alarge amount of a binder must be used in order to reduce theexfoliation.

However, in the case in which the large amount of binder is used,resistance may increase and voltage may decrease. Also, the mixing timeof a cathode active material composition may increase, and thus, processefficiency may decrease.

PRIOR ART DOCUMENTS Patent Document

KR 0809570 B1

DISCLOSURE OF THE INVENTION Technical Problem

Accordingly, the present invention is provided to solve the foregoingproblems.

An aspect of the present invention provides a cathode active materialincluding lithium transition metal phosphate capable of simultaneouslyimproving adhesion to a cathode collector and high outputcharacteristics.

Another aspect of the present invention provides a method of preparingthe cathode active material which may improve the processability of anelectrode during cathode preparation.

Another aspect of the present invention provides a cathode including thecathode active material and a method of preparing the cathode.

Another aspect of the present invention provides a lithium secondarybattery including the cathode.

Technical Solution

According to an aspect of the present invention, there is provided acathode active material including lithium transition metal phosphateparticles represented by Chemical Formula 1, wherein the lithiumtransition metal phosphate particles include a first secondary particleformed by agglomeration of two or more first primary particles; and asecond secondary particle formed by agglomeration of two or more secondprimary particles in the first secondary particle:

Li_(1+a)M1_(1−x)M2_(x)(PO_(4−b))X_(b)   Chemical Formula 1

in Chemical Formula 1,

M1 represents at least one element selected from the group consisting ofiron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc(Zn), and magnesium (Mg);

M2 represents at least one element selected from Groups 2 to 15 elementsexcluding M1;

X represents at least one element selected from the group consisting offluorine (F), sulfur (S), and nitrogen (N); and

−0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.

According to another aspect of the present invention, there is provideda method of preparing a cathode active material including: (i) preparingsecond secondary particles of lithium transition metal phosphate byprimary spray drying and primary sintering a first spray solutionincluding first lithium transition metal phosphate, a first carbonprecursor, and water in a spray chamber; and (ii) preparing firstsecondary particles of lithium transition metal phosphate respectivelyincluding the second secondary particle therein by secondary spraydrying and secondary sintering a second spray solution including thesecond secondary particles, second lithium transition metal phosphate, asecond carbon precursor, and water in a spray chamber.

According to another aspect of the present invention, there is provideda cathode including the cathode active material.

According to another aspect of the present invention, there is provideda method of preparing a cathode including: forming a cathode activematerial coating layer by coating a cathode collector with a cathodeactive material composition comprising the cathode active material and abinder mixed therein and drying the cathode collector; and rolling thecathode active material coating layer.

According to another aspect of the present invention, there is provideda lithium secondary battery including the cathode, an anode, and aseparator disposed between the cathode and the anode.

Advantageous Effects

In a cathode active material according to an embodiment of the presentinvention, since first primary particles and second primary particleshaving different average particle diameters are included, theexfoliation of the cathode active material from a cathode collector maybe minimized and performance characteristics, such as high outputcharacteristics and an increase in available capacity, of a secondarybattery may be further improved.

In addition, the first secondary particles are porous, the secondaryparticles are collapsed and fractured due to rolling when used in acathode. Thus, a spring back phenomenon may be reduced, and as a result,adhesion and processability may be further improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings attached to the specification illustratepreferred examples of the present invention by example, and serve toenable technical concepts of the present invention to be furtherunderstood together with detailed description of the invention givenbelow, and therefore the present invention should not be interpretedonly with matters in such drawings.

The FIGURE is a schematic cross-sectional view illustrating a cathodeactive material according to an embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail toallow for a clearer understanding of the present invention.

It will be understood that words or terms used in the specification andclaims shall not be interpreted as the meaning defined in commonly useddictionaries. It will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of theinvention, based on the principle that an inventor may properly definethe meaning of the words or terms to best explain the invention.

A cathode active material according to an embodiment of the presentinvention includes lithium transition metal phosphate particlesrepresented by the following Chemical Formula 1, wherein the lithiumtransition metal phosphate particles include a first secondary particleformed by agglomeration of two or more first primary particles, and asecond secondary particle formed by agglomeration of two or more secondprimary particles in the first secondary particle:

Li_(1+a)M1_(1−x)M2_(x)(PO_(4−b))X_(b)   Chemical Formula 1

in Chemical Formula 1,

M1 represents at least one element selected from the group consisting ofiron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc(Zn), and magnesium (Mg);

M2 represents at least one element selected from Groups 2 to 15 elementsexcluding M1;

X represents at least one element selected from the group consisting offluorine (F), sulfur (S), and nitrogen (N); and

−0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.

The present invention is for providing a cathode active materialincluding lithium transition metal phosphate which may simultaneouslyimprove adhesion to a cathode collector and high output characteristics.

In general, lithium transition metal phosphate having a small averageparticle diameter is required for a high-output cathode active materialincluding lithium transition metal phosphate. However, in the case thatthe cathode active material is composed of only the lithium transitionmetal phosphate having a small average particle diameter, the adhesionto the cathode collector may be reduced.

In contrast, lithium transition metal phosphate having a large averageparticle diameter may improve the adhesion. However, since electricalconductivity of the lithium transition metal phosphate having a largeaverage particle diameter may be low, internal resistance of a batterymay increase at high output levels when the lithium transition metalphosphate having a large average particle diameter is used as a cathodeactive material.

Thus, the present invention is for simultaneously addressing the abovelimitations.

That is, as illustrated in a schematic cross-sectional view of theFIGURE, lithium transition metal phosphate particles according to anembodiment of the present invention include a first secondary particleformed by agglomeration of two or more first primary particles, and asecond secondary particle formed by agglomeration of two or more secondprimary particles in the first primary particle.

According to an embodiment of the present invention, the first primaryparticles and the second primary particles may have different averageparticle diameters (D₅₀) for improving high output characteristics andcapacity characteristics of the secondary battery.

The average particle diameter of the first primary particle may begreater than the average particle diameter of the second primaryparticle.

That is, in the particles of the cathode active material according tothe embodiment of the present invention, the average particle diameterof the first primary particles constituting the first secondary particlethat may be in contact with the cathode collector may be in a range of200 nm to 500 nm. In order to increase the output characteristics andavailable capacity, the average particle diameter of the second primaryparticles constituting the second secondary particle that is formed inthe first secondary particle may be relatively smaller than that of thefirst primary particle, and for example, the average particle diameterthereof may be less than 50 nm, preferably, in a range of 10 nm to 45nm.

In the case that the average particle diameter of the first primaryparticles is excessively small, that is, less than 200 nm, the adhesionmay decrease and thus, process efficiency may not be obtained. In thecase in which the average particle diameter of the first primaryparticles is greater than 500 nm, formability of the lithium transitionmetal phosphate particles may be reduced and granulation may bedifficult to be controlled.

In the case that the average particle diameter of the second primaryparticles is greater than 50 nm, the improvement of high outputcharacteristics as desired in the present invention may be difficult.

Also, an average particle diameter of the first secondary particleformed by the agglomeration of the first primary particles may be in arange of 15 μm to 30 μm, and an average particle diameter of the secondsecondary particle formed by the agglomeration of the second primaryparticles may be in a range of 5 μm to 10 μm.

In the case that the average particle diameter of the first secondaryparticle is greater than 30 μm, since the mixing time for a uniformelectrode process may be excessively required, it may be difficult toobtain uniformity in the preparation of the electrode. In contrast, inthe case in which the average particle diameter of the first secondaryparticle is excessively small, that is, less than 15 μm, processefficiency may not be obtained.

In the present invention, the average particle diameter (D₅₀) of theparticles may be defined as a particle diameter at 50% in a cumulativeparticle diameter distribution. The average particle diameter (D₅₀) ofthe particles according to the embodiment of the present invention, forexample, may be measured by using a laser diffraction method. The laserdiffraction method may generally measure a particle diameter rangingfrom a submicron level to a few mm, and may obtain highly repeatable andhigh resolution results.

Also, according to an embodiment of the present invention, the firstsecondary particle may be porous as illustrated in the FIGURE. That is,the second primary particles may be formed in a shape in which thesecond primary particles are agglomerated in the first secondaryparticle, preferably in the core of the particle, and a porous portion(hollow portion) may be included between the first primary particles andthe second primary particles.

In the case that the first secondary particle is not porous and thefirst secondary particle and the second secondary particle are denselyformed to constitute the cathode active material, a spring backphenomenon of the secondary particle may occur when the cathode activematerial is used in a cathode.

Herein, the expression “spring back phenomenon” denotes an elasticphenomenon in which an object attempts to restore its original statewhen the object is deformed by applying a pressure and the pressure isthen removed.

That is, in the case that a cathode is prepared by coating a cathodecollector with a cathode active material composition including lithiumtransition metal phosphate particles and rolling the cathode collector,the spring back phenomenon for recovering the original shape ofspherical secondary particles may occur after the rolling when the firstsecondary particle and second secondary particle of the lithiumtransition metal phosphate particles are dense, and as a result, pores(gaps) between the particles may be formed. Since the adhesion to thecathode collector may be reduced due to the formation of the pores,exfoliation may occur. Also, since the pores may function as resistance,electrical conductivity of the electrode may decrease.

In order to address the above limitations, a large amount of a bindermust be used. However, when the large amount of binder is used,resistance may increase and voltage may decrease.

Since the first secondary particle of the cathode active materialaccording to the embodiment of the present invention is porous, inparticular, the porous portion is formed between the first primaryparticles and the second primary particles, the first secondary particleand the second secondary particle of the cathode active material arecollapsed and fractured during the rolling process when the cathodeactive material is used in the cathode. Thus, the shape of the secondaryparticles may disappear and the secondary particles may become primaryparticles. Accordingly, the spring back phenomenon may be reduced and asa result, the adhesion and process efficiency may be improved.

According to an embodiment of the present invention, an internalporosity of the cathode active material may be in a range of 30% to 40%.

In the case that the internal porosity of the cathode active material isless than 30%, since a space may be insufficient in which the secondaryparticles are collapsed due to the porosity during the rolling processof the electrode to become primary particles, the desired effect of thepresent invention cannot be obtained. In the case in which the internalporosity of the cathode active material is greater than 40%, a volume ofthe battery may be increased due to the relatively increased volume ofthe electrode, and as a result, capacity per volume may decrease.

According to an embodiment of the present invention, the internalporosity of the cathode active material may be defined below:

Internal porosity=volume of pores per unit mass/(specific volume+volumeof pores per unit mass)

The measurement of the internal porosity is not particularly limited.According to an embodiment of the present invention, the internalporosity, for example, may be measured by using absorption gas, such asnitrogen, and BELSORP (BET instrument) by BEL Japan, Inc.

The lithium transition metal phosphate according to the embodiment ofthe present invention has an olivine-type structure and may be an oxiderepresented by Chemical Formula 1.

Also, the lithium transition metal phosphate may be lithium ironphosphate represented by Chemical Formula 2 below and for example, maybe LiFePO₄:

Li_(1+a)Fe_(1−x)M_(x)(PO_(4−b))X_(b)   Chemical Formula 2

in Chemical Formula 2,

M represents at least one selected from the group consisting of aluminum(Al), Mg, Ni, Co, Mn, titanium (Ti), gallium (Ga), Cu, vanadium (V),niobium (Nb), Zirconium (Zr), cerium (Ce), indium (In), Zn, and yttrium(Y),

X represents at least one selected from the group consisting of F, S,and N, and

−0.5≦a≦+0.5, 0≦x≦0.5, and 0≦b≦0.1.

The cathode active material according to the embodiment of the presentinvention may further include a carbon coating layer on the firstprimary particles and the second primary particles, and as a result, theelectrical conductivity may be further improved.

The carbon coating layer may include saccharides, and the saccharidesmay be obtained using any one selected from the group consisting ofglucose, fructose, galactose, sucrose, maltose, and lactose, or amixture of two or more thereof.

A thickness of the carbon coating layer is in a range of 5 nm to 100 nmand may be in a range of 5 nm to 50 nm. In the case that the thicknessof the carbon coating layer is less than 5 nm, an effect of increasingthe electrical conductivity due to the carbon coating layer may beinsignificant. In contrast, in the case in which the thickness of thecarbon coating layer is greater than 100 nm, mobility of lithium ions isreduced, and thus, the resistance may increase.

In the cathode active material according to the embodiment of thepresent invention, an amount of carbon included in the carbon coatinglayer is in a range of 2 wt % to 70 wt %, may be in a range of 3 wt % to50 wt %, and for example, may be in a range of 4 wt % to 40 wt % basedon a total weight of the cathode active material.

In the case that the amount of carbon is greater than wt %, since athick coating layer is formed due to the excessive amount of carbon, anadditional irreversible reaction may occur. Thus, a discharge capacitymay be significantly decreased. In contrast, in the case in which theamount of carbon is less than 2 wt %, since the carbon coating layer maybecome excessively thin, the effect of increasing the electricalconductivity may be insignificant. When the carbon coating layer isfurther included, the cathode active material may be coated with thecarbon coating layer by being surrounded by the carbon coating layer.

In order to obtain excellent electrical conductivity, stability ofcrystal structure, and high bulk density even in the case in which thefirst secondary particle and the second secondary particle are collapsedto return to the primary particles during the preparation of the cathodein the present invention, it is desirable to form the first and secondsecondary particles by using first and second primary particles in acrystallized state. That is, the primary particles may eachindependently have an olivine-type crystal structure.

Also, in terms of facilitating the return to the first and secondprimary particles while the first and second secondary particles arecollapsed, the primary particles may form the secondary particles byagglomeration due to physical bonds, such as van der Waals forces,instead of chemical bonds such as covalent bonds or ionic bonds.

A method of preparing the cathode active material according to anembodiment of the present invention may include (i) preparing secondsecondary particles of lithium transition metal phosphate by primaryspray drying and primary sintering a first spray solution includingfirst lithium transition metal phosphate, a first carbon precursor, andwater in a spray chamber; and (ii) preparing first secondary particlesof lithium transition metal phosphate respectively including the secondsecondary particle therein by secondary spray drying and secondarysintering a second spray solution including the second secondaryparticles, second lithium transition metal phosphate, a second carbonprecursor, and water in a spray chamber.

According to an embodiment of the present invention, in the cathodeactive material, the secondary particles of the lithium transition metalphosphate particles may be formed by a separate granulation processafter the preparation of the primary particles. Also, the secondaryparticles may be prepared by a method of forming primary particles andsimultaneously agglomerating the primary particles though a singleprocess.

Hereinafter, the method of preparing the cathode active materialaccording to the embodiment of the present invention will be describedusing a spray drying method as an example.

When the method of preparing the cathode active material according tothe embodiment of the present invention is specifically examined, step(i) may be a step of preparing second secondary particles of lithiumtransition metal phosphate, wherein a first spray solution includingfirst lithium transition metal phosphate, a first carbon precursor, andwater is subjected to primary spray drying and primary sintering in aspray chamber to prepare the second secondary particles of lithiumtransition metal phosphate.

Specifically, the lithium transition metal phosphate is an oxide ofChemical Formula 1, may be an oxide of Chemical Formula 2, and forexample, may be LiFePO₄.

The lithium transition metal phosphate may be prepared using a methodtypically used in the art, and for example, may be prepared by mixing alithium-containing precursor and a transition metal-containingprecursor.

The lithium-containing precursor may be any one selected from the groupconsisting of Li₂CO₃, Li(OH), Li(OH)·H₂O, and LiNO₃, or a mixture of twoor more thereof.

A precursor that is typically used and contains transition metal may beused as the lithium-containing precursor, and in particular, thelithium-containing precursor may include an iron (Fe)-containingprecursor and a phosphorous (P)-containing precursor.

The Fe-containing precursor is a ferrous compound and may be any oneselected from the group consisting of FeSO₄, FeSO₄·7H₂O, FeC₂O₄·2H₂O,and FeCl₂, or a mixture of two or more thereof.

The P-containing precursor may be any one selected from the groupconsisting of H₃PO₄, NH₄H₂PO₄, (NH₄)₂HPO₄, and P₂O₅, or a mixture of twoor more thereof.

Also, an alkalizing agent may be further included to adjust a pH valueduring the preparation of the lithium transition metal phosphateprecursor. The alkalizing agent may be any one selected from the groupconsisting of alkali metal hydroxides, alkaline earth metal hydroxides,and ammonia compounds, or a mixture of two or more thereof.

A typically used spray dryer may be used as the above spray dryer, andfor example, an ultrasonic spray dryer, an air nozzle spray dryer, anultrasonic nozzle spray dryer, a filter expansion aerosol generator, oran electrostatic spray dryer may be used. However, the present inventionis not limited thereto.

According to an embodiment of the present invention, the first lithiumtransition metal phosphate may be used in an amount of 5 parts by weightto 40 parts by weight based on 100 parts by weight of the water.

The spray solution may be sprayed through a disc rotating at a highspeed in the chamber, and the spraying and the drying may be performedin the same chamber.

Also, according to an embodiment of the present invention, since thefirst spray solution may include the first carbon precursor as well asthe first lithium transition metal phosphate and water in step (i), acarbon coating layer may be further included on the second primaryparticles.

Saccharides, for example, may be used as the first carbon precursoraccording to an embodiment of the present invention, and specificexamples of the saccharides may be any one selected from the groupconsisting of glucose, fructose, galactose, sucrose, maltose, andlactose, or a mixture of two or more thereof.

According to an embodiment of the present invention, in the method ofpreparing the cathode active material, a total solid content (TSC) ofthe second secondary particles may be in a range of 25% to 40%.

In the present invention, the expression “total solid content (TSC)”denotes a solid material remained after evaporation of water in a totalweight of the spray solution including the lithium transition metalphosphate and water, that is, a value that is obtained by converting thesecondary particles obtained after the drying to a percentage.

According to an embodiment of the present invention, in the case thatthe TSC is less than 25%, the average particle diameter of the secondsecondary particles may decrease and productivity may decrease. In thecase in which the TSC is greater than 40%, there may be limitations inthe control of the average particle diameter of the secondary particlesand the preparation of the electrode.

The TSC of the lithium transition metal phosphate particles may berealized by controlling spray drying conditions, for example, flow andflow rate of carrier gas, sintering temperature, retention time in areactor, and internal pressure.

Also, according to an embodiment of the present invention, the spraydrying temperature and sintering temperature may be important forcontrolling the degree of porosity.

According to an embodiment of the present invention, the primary spraydrying may be performed in a temperature range of 20° C. to 350° C., butit is advantageous to perform the primary spray drying at a temperatureas low as possible for the densification of the second secondaryparticles.

Furthermore, the primary sintering may be performed in a temperaturerange of 150° C. to 300° C. The sintering, for example, may be performedin an atmosphere of an inert gas such as argon (Ar) or nitrogen (N₂).

In the method of preparing the cathode active material according to theembodiment of the present invention, step (ii) may be a step ofpreparing first secondary particles respectively including the secondsecondary particle therein, wherein a second spray solution includingthe second secondary particles, second lithium transition metalphosphate, a second carbon precursor, and water is subjected tosecondary spray drying and secondary sintering in a spray chamber toprepare the first secondary particles of lithium transition metalphosphate respectively including the second secondary particle therein.

According to an embodiment of the present invention, a ratio of thesecond secondary particles to the second lithium transition metalphosphate is in a range of 1:0.3 to 1:5 as a weight ratio, and may be ina range of 1:0.5 to 1:2 as a weight ratio. In the case that the amountof the second secondary particles used is less than the above range, theadhesion to the electrode may decrease, and in the case in which theamount of the second secondary particles used is greater than the aboverange, electrode resistance may increase and output may decrease.

Also, the second lithium transition metal phosphate may be used in anamount of 1 part by weight to 20 parts by weight based on 100 parts byweight of the water.

The second spray solution may be sprayed through a disc rotating at ahigh speed in the chamber, and the spraying and the drying may beperformed in the same chamber.

According to an embodiment of the present invention, a flow rate of thesecond spray solution may be in a range of 10 ml/min to 90 ml/min. Inthe case that the flow rate is less than 10 ml/min, a cloggingphenomenon during the introduction of the second spray solution into thespray dryer and a decrease in productivity may occur. In the case inwhich the flow rate is greater than 90 ml/min, the lithium transitionmetal particles may not be dried due to water condensation in the spraydryer.

Also, according to an embodiment of the present invention, since thesecond spray solution may include the second carbon precursor as well asthe second lithium transition metal phosphate and water in step (ii), acarbon coating layer may be further included on the first primaryparticles.

The saccharides described in the case of the first carbon precursor maybe used as the second carbon precursor according to an embodiment of thepresent invention, and the second carbon precursor may be used in anamount of 1 part by weight to 40 parts by weight based on 100 parts byweight of the lithium transition metal phosphate. In this case, the typeand used amount of the first carbon precursor and the second carbonprecursor may be the same or different from each other.

According to an embodiment of the present invention, in the method ofpreparing the cathode active material, a TSC of the first secondaryparticles may be in a range of 10% to 20%.

Since the range of the TSC of the first secondary particles issatisfied, the first secondary particle may be porous, and the secondprimary particles may be formed in a shape in which the second primaryparticles are agglomerated in the first secondary particle, preferablyin the core of the particle.

According to an embodiment of the present invention, in the case thatthe TSC is less than 10%, the average particle diameter of the secondaryparticles may decrease and the productivity may decrease. In the case inwhich the TSC is greater than 20%, there may be limitations in theproductivity while the average particle diameter of the secondaryparticles increases.

The TSC of the second lithium transition metal phosphate and the firstsecondary particles may be realized by controlling spray dryingconditions, for example, flow and flow rate of carrier gas, sinteringtemperature, retention time in a reactor, and internal pressure.

According to an embodiment of the present invention, the secondary spraydrying may be performed in a temperature range of 20° C. to 350° C., butit is advantageous to perform the secondary spray drying at atemperature as low as possible for the densification of the firstsecondary particles.

Furthermore, the secondary sintering may be performed in a temperaturerange of 700° C. to 800° C. The secondary sintering, for example, may beperformed in an atmosphere of an inert gas such as Ar or N₂.

Also, in the method of preparing the cathode active material accordingto the embodiment of the present invention, the first and second spraysolutions may further include a cellulose-based dispersant to increasethe dispersion thereof.

A cellulose-based dispersant typically used in the art may be used asthe cellulose-based dispersant and for example, may include any oneselected from the group consisting of carboxymethyl cellulose,carboxyethyl cellulose, aminoethyl cellulose, and oxyethyl cellulose, ora mixture of two or more thereof.

The dispersant may be used in an amount of 1 part by weight to 10 partsby weight based on 100 parts by weight of the lithium transition metalphosphate.

The present invention provides a cathode including the cathode activematerial.

According to an embodiment of the present invention, a method ofpreparing the cathode may include: forming a cathode active materialcoating layer by coating a cathode collector with a cathode activematerial composition comprising the cathode active material and a bindermixed therein and drying the cathode collector; and rolling the cathodeactive material coating layer.

According to an embodiment of the present invention, the rolling, forexample, may be performed using a roll press by a typical method used inthe art.

Also, in the cathode active material according to the embodiment of thepresent invention, since the rolling is performed on the first secondaryparticles, the secondary particles may not be picked up by a rollerduring the rolling according to the appropriate diameter of thesecondary particles. Thus, processability of the electrode may beimproved.

That is, according to an embodiment of the present invention, since thefirst secondary particles and the second secondary particles arefractured by the roll press during the rolling when the cathode activematerial is used in the cathode, the shape of the first secondaryparticles and the second secondary particles may disappear and the firstsecondary particles and the second secondary particles may respectivelybecome primary particles.

According to an embodiment of the present invention, thicknesses of thecathode active material coating layer before and after the rolling maybe different from each other.

According to an embodiment of the present invention, the thickness ofthe cathode active material coating layer after the rolling may besmaller than the thickness of the cathode active material coating layerbefore the rolling.

For example, the thickness of the cathode active material coating layerbefore the rolling may be in a range of 80 μm to 200 μm, and thethickness of the cathode active material coating layer after the rollingmay be in a range of 20 μm to 80 μm.

Also, the thickness of the cathode active material coating layer afterthe rolling may be smaller than the thickness of the cathode activematerial coating layer before the rolling, and for example, may be in arange of 45 μm to 70 μm. However, the present invention is not limitedthereto.

The binder is a component that assists the binding between an activematerial and a conductive agent, and the binding with a currentcollector. The binder is typically added in an amount of 1 wt % to 30 wt% based on a total weight of the cathode active material composition.Examples of the binder may be any one selected from the group consistingof a polyvinylidene fluoride-hexafluoropropylene copolymer(PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile,polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose(CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,polyacrylate, an ethylene-propylene-diene monomer (EPDM), a sulfonatedEPDM, a styrene-butadiene rubber (SBR), and a fluorine rubber, or amixture of two or more thereof.

The conductive agent may be typically added in an amount of 0.05 wt % to5 wt % based on the total weight of the cathode active materialcomposition. Any conductive agent may be used without particularlimitation so long as it has suitable conductivity without causing sidereactions with other elements of the battery. For example, theconductive agent may include a conductive material such as: graphitesuch as natural graphite and artificial graphite; carbon black such assuper-p, acetylene black, Ketjen black, channel black, furnace black,lamp black, and thermal black; conductive fibers such as carbon fibersand metal fibers; metal powder such as fluorocarbon powder, aluminumpowder, and nickel powder; conductive whiskers such as zinc oxidewhiskers and potassium titanate whiskers; conductive oxide such astitanium oxide; or polyphenylene derivatives.

A lithium secondary battery according to an embodiment of the presentinvention includes the cathode, an anode, and a separator disposedbetween the cathode and the anode.

In the anode, a carbon material, lithium metal, silicon, or tin, whichmay intercalate and deintercalate lithium ions, may be typically used asan anode active material. For example, the carbon material may be usedand both low crystalline carbon and high crystalline carbon may be usedas the carbon material. Typical examples of the low crystalline carbonmay be soft carbon and hard carbon, and typical examples of the highcrystalline carbon may be natural graphite, Kish graphite, pyrolyticcarbon, mesophase pitch-based carbon fibers, meso-carbon microbeads,mesophase pitches, and high-temperature sintered carbon such aspetroleum or coal tar pitch derived cokes.

Similar to the cathode, any binder typically used in the art may be usedas a binder for the anode. An anode active material and the aboveadditives are mixed and stirred to prepare an anode active materialcomposition. Then, a current collector is coated therewith and pressedto prepare the anode.

Also, a typical porous polymer film used as a typical separator, forexample, a porous polymer film prepared from a polyolefin-based polymer,such as an ethylene homopolymer, a propylene homopolymer, anethylene/butene copolymer, an ethylene/hexene copolymer, and anethylene/methacrylate copolymer, may be used alone or in a laminationtherewith as the separator. Also, a typical porous nonwoven fabric, forexample, a nonwoven fabric formed of high melting point glass fibers orpolyethylene terephthalate fibers may be used. However, the presentinvention is not limited thereto.

A shape of the lithium secondary battery of the present invention is notparticularly limited, and for example, a cylindrical type using a can, aprismatic type, a pouch type, or a coin type may be used.

Hereinafter, the present invention will be described in detail,according to specific examples. The invention may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein. Rather, these example embodiments areprovided so that this description will be thorough and complete, andwill fully convey the scope of the present invention to those skilled inthe art.

Hereinafter, the present invention will be described in more detail,according to examples and experimental examples. However, the presentinvention is not limited thereto.

Preparation of Cathode Active Material Example 1

Step (i) Preparation of Second Secondary Particles

A mixed solution was prepared in which ammonia water was added to amixture of 1.5 mole of lithium aqueous solution (LiOH·H₂O) and a ferroussulfate aqueous solution containing 0.5 mole of iron sulfate(FeSO₄·7H₂O), 0.55 mole of phosphoric acid (P₂O₅), and an antioxidant toadjust a pH value in a range of 5.5 to 7. The mixed solution wasintroduced at a constant rate through a continuous supercritical reactorunder conditions of a temperature of about 375° C. to about 450° C. anda pressure of 250 bar to 300 bar to prepare a LiFePO₄ solution aslithium iron phosphate during a reaction time of a few seconds.

The LiFePO₄ solution was filtered to obtain lithium iron phosphateparticles. Purity of the lithium iron phosphate particles was analyzedby x-ray diffraction (XRD) analysis and primary particles were analyzedby a scanning electron microscope (SEM).

The washed lithium iron phosphate was reslurried in distilled water, andsucrose as a first carbon precursor was then added to the above solutionin an amount of 4 parts by weight based on 100 parts by weight of thelithium iron phosphate to obtain a first spray solution. The first spraysolution including the lithium iron phosphate solution having thesucrose added thereto was provided into a chamber of a spray dryer (byEIN SYSTEMS, Co., Ltd.). Then, the first spray solution was sprayed inthe chamber and dried. In this case, the spray drying was performedunder conditions including a drying temperature of 130° C., an internalpressure of −20 mbar, and a flow rate of 45 ml/min, and second secondaryparticles formed by the agglomeration of second primary particles oflithium iron phosphate (LiFePO₄) were then obtained by primary sinteringat 200° C. in air.

Step (ii) Preparation of First Secondary Particles RespectivelyIncluding the Second Secondary Particle Therein

The second secondary particles, lithium iron phosphate, carboxymethylcellulose as a dispersant, sucrose as a second carbon precursor, andpure water were mixed and stirred to obtain a second spray solution.

In this case, a weight ratio of the second secondary particles to thelithium iron phosphate was 1:1, and the lithium iron phosphate was usedin an amount of about 20 parts by weight based on 100 parts by weight ofthe pure water. Also, the sucrose and the carboxymethyl cellulose wererespectively used in an amount of 4 parts by weight based on 100 partsby weight of the lithium iron phosphate.

The second spray solution was provided into a chamber of a spray dryer(by EIN SYSTEMS, Co., Ltd.). Then, the second spray solution was sprayedin the chamber and dried. In this case, the spray drying was performedunder conditions including a drying temperature of 95° C. to 115° C., aninternal pressure of −20 mbar, and a flow rate of 40 ml/min, and firstsecondary particles respectively including the second secondary particletherein and formed by the agglomeration of first primary particles werethen obtained by secondary sintering at 700° C. in air.

Comparative Example 1

A mixed solution was prepared in which ammonia water was added to amixture of 1.5 mole of lithium aqueous solution (LiOH·H₂O) and a ferroussulfate aqueous solution containing 0.5 mole of iron sulfate(FeSO₄·7H₂O), 0.55 mole of phosphoric acid (P₂O₅), and an antioxidant toadjust a pH value in a range of 5.5 to 7. The mixed solution wasintroduced at a constant rate through a continuous supercritical reactorunder conditions of a temperature of about 375° C. to about 450° C. anda pressure of 250 bar to 300 bar to prepare a LiFePO₄ solution aslithium iron phosphate during a reaction time of a few seconds.

The LiFePO₄ solution was filtered to obtain lithium iron phosphateparticles. Purity of the lithium iron phosphate particles was analyzedby XRD analysis and primary particles were analyzed by a SEM.

The washed lithium iron phosphate was reslurried in distilled water, andsucrose as a first carbon precursor was then added to the above solutionin an amount of 4 parts by weight based on 100 parts by weight of thelithium iron phosphate to obtain a spray solution. The spray solutionincluding the lithium iron phosphate solution having the sucrose addedthereto was provided into a chamber of a spray dryer (by EIN SYSTEMS,Co., Ltd.). Then, the spray solution was sprayed in the chamber anddried. In this case, the spray drying was performed under conditionsincluding a drying temperature of 130° C., an internal pressure of −20mbar, and a flow rate of 15 ml/min, and lithium iron phosphate havingsecondary particles, which were formed by the agglomeration of primaryparticles of lithium iron phosphate (LiFePO₄), was then obtained bysintering at 700° C. in air.

TABLE 1 Average Average Average Average particle particle particleparticle diameter¹ diameter¹ diameter¹ diameter¹ of first of first ofsecond of second primary secondary primary secondary Internal particlesparticles particles particles porosity² Examples (nm) (μm) (nm) (μm) (%)Example 1 220 16 40 9 35 Comparative 40 15 x x 35 Example 1

1. Average particle diameter: laser diffraction method (Microtac MT3000)

2. Internal porosity=volume of pores per unit mass/(specificvolume+volume of pores per unit mass)

(use BELSORP (BET instrument) by BEL Japan Inc., use values calculatedby the Barrett-Joyner-Halenda (BJH) method, i.e., a mesopore measurementmethod)

Preparation of Lithium Secondary Battery Example 2

Cathode Preparation

A cathode active material composition was prepared by adding 90 wt % ofthe cathode active material prepared in Example 1, 5 wt % of carbonblack as a conductive agent, and 5 wt % of polyvinylidene fluoride(PVdF) as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. Anabout 20 μm thick aluminum (Al) thin film as a cathode collector wascoated with the cathode active material composition and dried, and theAl thin film was then roll-pressed to prepare a cathode.

Anode Preparation

An anode active material composition was prepared by mixing 96.3 wt % ofcarbon powder as an anode active material, 1.0 wt % of super-p as aconductive agent, and 1.5 wt % of styrene-butadiene rubber (SBR) and 1.2wt % of carboxymethyl cellulose (CMC) as a binder, and adding themixture to NMP as a solvent. A 10 μm thick copper (Cu) thin film as ananode collector was coated with the anode active material compositionand dried, and the Cu thin film was then roll-pressed to prepare ananode.

Non-Aqueous Electrolyte Solution Preparation

A 1 M LiPF₆ non-aqueous electrolyte solution was prepared by addingLiPF₆ to a non-aqueous electrolyte solvent that was prepared by mixingethylene carbonate and diethyl carbonate, as an electrolyte, at a volumeratio of 30:70.

Lithium Secondary Battery Preparation

A mixed separator of polyethylene and polypropylene was disposed betweenthe cathode and anode thus prepared, and a polymer type battery was thenprepared by a typical method. Then, the preparation of each lithiumsecondary battery was completed by injecting the prepared non-aqueouselectrolyte solution.

Comparative Example 2

A lithium secondary battery was prepared in the same manner as inExample 1 except that the cathode active material prepared inComparative Example 1 was used.

Experimental Example 1 Charge and Discharge Characteristics

Charge and discharge characteristics of the lithium secondary batteriesobtained in Example 2 and Comparative

Example 2 were measured. In the initial 6^(th) cycle, the lithiumsecondary batteries prepared in Example 2 and Comparative Example 2 werecharged at 0.2 C to 4.2 V under constant current/constant voltage(CC/CV(2%)) conditions. Thereafter, the lithium secondary batteries weredischarged at a CC of 2.0 C to a voltage of 2.5 V vs. Li/Li⁺, and thecharge and discharge were then terminated. The results thereof arepresented in Table 2 below.

Experimental Example 2 Adhesion Measurement

Adhesion measurement was performed on the cathodes prepared in Example 2and Comparative Example 2. The adhesion measurement was performed usinga generally known 180-degree peel test. The results thereof arepresented in Table 2 below.

TABLE 2 Capacity retention Examples ratio (2 C/0.1 C) (%) Adhesion (gf)Example 2 92 22 Comparative Example 2 90 9

As illustrated in Table 2, a capacity retention ratio of Example 2 ofthe present invention was 92% and a capacity retention ratio ofComparative Example 2 was 90%. Thus, it may be understood that thecapacity retention ratio of Example 2 was improved by about 2% incomparison to that of Comparative Example 2.

With respect to the cathode obtained in Example 2, adhesion to thecathode collector was 22 gf, and with respect to the cathode obtained inComparative Example 2, adhesion to the cathode collector was 9 gf. Thus,it may be understood that the adhesion of Example 2 was improved by 10times or more in comparison to that of Comparative Example 2.

INDUSTRIAL APPLICABILITY

In a cathode active material according to an embodiment of the presentinvention, since first primary particles and second primary particleshaving different average particle diameters are included, theexfoliation of the cathode active material from a cathode collector maybe minimized and performance characteristics, such as high outputcharacteristics and an increase in available capacity, of a secondarybattery may be further improved. Thus, the cathode active material maybe suitable for a secondary battery.

1. A method of preparing a cathode active material, the methodcomprising: (i) preparing second secondary particles of lithiumtransition metal phosphate by primary spray drying and primary sinteringa first spray solution including first lithium transition metalphosphate, a first carbon precursor, and water in a spray chamber; and(ii) preparing first secondary particles of lithium transition metalphosphate respectively including the second secondary particle thereinby secondary spray drying and secondary sintering a second spraysolution including the second secondary particles, second lithiumtransition metal phosphate, a second carbon precursor, and water in aspray chamber, wherein the first secondary particle is formed byagglomeration of two or more first primary particles, and the secondsecondary particle is formed by agglomeration of two or more secondprimary particles, and wherein the cathode active material furthercomprises a carbon coating layer on the first primary particles and thesecond primary particles.
 2. The method of claim 1, wherein a totalsolid content (TSC) of the second secondary particles is in a range of25% to 40%.
 3. The method of claim 1, wherein a TSC of the firstsecondary particles is in a range of 10% to 20%.
 4. The method of claim1, wherein a ratio of the second secondary particles to the secondlithium transition metal phosphate is in a range of 1:0.3 to 1:5 as aweight ratio.
 5. The method of claim 1, wherein the primary sintering isperformed in a temperature range of 150° C. to 300° C.
 6. The method ofclaim 1, wherein the secondary sintering is performed in a temperaturerange of 700° C. to 800° C.
 7. The method of claim 1, wherein the secondspray solution is spray-dried in a flow rate range of 10 ml/min to 90ml/min.
 8. The method of claim 1, wherein the primary spray drying andthe secondary spray drying are performed in a temperature range of 20°C. to 350° C.
 9. The method of claim 1, wherein the first and secondcarbon precursors are saccharides.
 10. The method of claim 9, whereinthe saccharides comprise a solution including any one selected from thegroup consisting of glucose, fructose, galactose, sucrose, maltose, andlactose, or a mixture of two or more thereof.
 11. The method of claim 1,wherein the first and second carbon precursors are used in an amount of1 part by weight to 40 parts by weight based on 100 parts by weight ofthe lithium transition metal phosphate.
 12. The method of claim 1,wherein the first and second spray solutions further comprise acellulose-based dispersant.
 13. The method of claim 12, wherein thecellulose-based dispersant comprises any one selected from the groupconsisting of carboxymethyl cellulose, carboxyethyl cellulose,aminoethyl cellulose, and oxyethyl cellulose, or a mixture of two ormore thereof.
 14. The method of claim 12, wherein the dispersant is usedin an amount of 1 part by weight to 10 parts by weight based on 100parts by weight of the lithium transition metal phosphate.